VirusesChapter 18.2

Pages 525 - 531

How were Viruses

Discovered?• Late 1800’s - bacteria known to causes disease• Scientists found tobacco plants were becoming

diseased.• Scientists collected samples of the diseased

plant and filtered it through pores small enough to catch bacteria.

• Observed that the filtered fluid still caused disease in tobacco plants

• Scientists infer the disease is not caused by bacteria, but by something small enough to get through the filter.

• This pathogen later named the “Tobacco Mosaic Virus” (TMV)

A tobacco plant suffering from

“Tobacco Mosaic Virus”

Is a Virus Alive?• Viruses are NOT considered living

because they do not display the 8 characteristics of life:– They do not grow– They cannot reproduce on their own– They don’t carry out metabolic activities– They do not maintain homeostasis

• Though not alive, viruses have a major impact on the world.

Viral Structure• Nucleic Acids

– Can be DNA or RNA– Code for components of the virus

• Capsid– Protein coat that houses genetic material

• Envelope– Gives a virus its overall spherical shape which can

help a virus enter a cell

• Tail Fibers– Act like a syringe which inject viral DNA into host

• All viruses have nucleic acid and a capsid. Only some viruses have an envelope and tail fibers.

Bacteriophage• A virus that infects bacteria

Protein Coat

Viral DNA

Bacterial Cell

Tail Fibers

(Capsid)

Viral Replication• Viral infection begins when a virus’ genetic material

enters a host cell.• Then it can either enter the lytic or lysogenic cycle• Lytic Cycle

– Viral infection, reproduction, and cell destruction– Virus uses host cells “stuff” to make more of itself– Cell breaks open, and new viruses are released to infect more

cells.

• Lysogenic Cycle (“gentle”)– When viral DNA becomes part of its host

cell’s DNA (prophage)– Viral DNA replicates with host cell’s DNA– Under stressful situations, virus may enter lytic cycle

Click here to

see them both

in action!

Animation for Lytic and Lysogenic Cycles

•http://highered.mcgraw-hill.com/sites/0072995246/student_view0/chapter8/lambda_phage_replication_cycle.html

Viruses and Humans• Viruses cause most common respiratory illnesses,

such as colds and coughs.• They can cause a variety of infections from diarrhea

to meningitis.• It may take up to 2-3 weeks for your body to resolve a

viral infection completely.• Antibiotics DO NOT work to

fight viruses!• Viral infection can be

prevented by vaccination.

How was Vaccination Discovered?

• Edward Jenner noticed that the milk-maids who contracted cow pox (not so serious) didn’t contract small pox (very serious!)

• He inoculated a healthy boy with cow pox (don’t try this at home!) to test his hypothesis that getting cow pox can prevent getting small pox.

• HE WAS RIGHT! (phew!)

How does Vaccination

Work?• Vaccination is the process of using one virus to

immunize against another virus.• Vaccines are usually a very mild dose of the

disease-causing virus which has been weakened or killed.

• These vaccines stimulate the creation of antibodies in the body that recognize and attack a particular infection.